The image to the left is a pseudo streak imageof the flow of a highly
elastic solution through an axisymmetric 4:1:4 contraction-expansion. The
fluid is a 0.025wt% high molecular weight monodispersepolystyrene dissolved
in an oligomeric polystyrene. To view a movie showingthe growth of the
enhanced upstream vortex structure and the onset of anelastic instability
click on the avi file below.

Notes to Aid Numerical Simulations
We have generated a series of notes entitled ViscoelasticFlows in Abrubt
Contraction-Expansions which should provide the necessary information
for those interested in numerically simulating this complexflow. These
notes include a complete description of the shear andtransient extensional
rheology of our test fluid (Fluid_Rheology.pdf),
a discussion of how we chose the proper relaxation time (Relaxation_Time.pdf),
the specifications for our test geometry (Geometry.pdf)
and the definition of the dimensionless pressure drop we report (Pressure_Drop.pdf).
Please also see our article entitled Extensional Flow of a
PolystyreneBoger Fluid Through a 4:1:4 Axisymmetric Contraction/Expansion
recently published in the Journal of Non-Newtonian Fluid Mechanics (GHM38.pdf).

Stretching and Breakup of Polymeric Liquids in a
Microfilament Rheometer

A microfilament rheometer (MFR) that can be used to readily differentiate
between the response of different fluid formulations.The device relies on a
detailed observation of the rate of extensionalthinning of a Newtonian or a
viscoelastic fluid filament and provides adirect measurement of the ultimate
time to break-up of the fluid filament. Measurements are performed in
a controlled temperature and environmentalconditions. We consider four
different classes of entangled polymer liquids that are of importance
commercially including (i) pressure sensitive adhesives,(ii) branched and
linear polymer melts, (iii) concentrated polymer solutionsand (iv) aqueous
solutions of associating polymers such as HEUR (Hydrophobically modified
urethane-ethoxylate). We observed that the breakup dynamics in these liquids
depend on the extensional viscosity, on molecular parameterssuch as the
chain-length, entanglement density or degree of chain branching,and on
external factors such as solvent volatility. Varying these factors changes
the dominant time scales in the extensional flow.

For example, pressure sensitive adhesives are elastically stabilized against
breakupand are 'tacky' if the solvent evaporation rate l-1evap=
h/R0 (where h is the mass transfer
coefficient for the solvent and R0 is the
initial radiusof the filament) is significantly larger than the stress
relaxation ratel-1whereas they are
'non-tacky' and undergo capillary breakup if the evaporation rate is much
slower than the capillary necking rate l-1neck=
s /h0R0)( where
s is the surface tension and
h0is the zero
shear viscosity). In the case of associative polymers, the breakup mechanism
also depends on the concentration of micelles and theionic surfactant
strength.

The Collapse of Highly Viscous Bubbles In this video a
high molecular weight (high viscosity) Poly(dimethyl siloxane) polymer has a
gas bubble insterted into it. The free bubble at the surface is then
rupturedfrom above using a pin. Due to the high viscosity of the
silicone oil film drainage is not possible and hence the bubble "skin" must
collapse under gravity. The stresses introduced result in the clearly
visible buckling of the skin. Note the bubble is viewed from above.
bubble.avi (1.4 MB)